Specific binding and uptake of opioids in brain slices

Studies on slices from whole rat brain indicate that the opioid peptide/receptor, but not the opiate drug/receptor, complex is internalized by metabolically-dependent processes. Opiate drugs displace 3H-etorphine from high affinity binding sites with potencies identical to those in brain homogenates. 3H-metenkephalin (ME) binds to a high affinity (IC50 10 nM) and a low affinity (micromolar) site. Opiate drugs and beta-endorphin compete at the high affinity but not at the low affinity binding site for ME. The biological relevance of the low affinity ME-binding site, which like the high affinity site is internalized, remains to be determined. The slice technique should be useful in the characterization of receptor dynamics that may be altered during opiate tolerance and dependence.